Fitting Together the H I Absorption and Emission in the Southern Galactic Plane Survey
Dickey, JM and McClure-Griffiths, NM and Gaensler, BM and Green, AJ, Fitting Together the H I Absorption and Emission in the Southern Galactic Plane Survey, Astrophysical Journal, 585, (2) pp. 801-822. ISSN 0004-637X (2003) [Refereed Article]
In this paper we study 21 cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326° < l < 333°) of the Southern Galactic Plane Survey. This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single-dish telescope. In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner Galaxy. The resulting mean opacity of the H I, 〈k〉, is measured as a function of Galactic radius; it increases going in from the solar circle, to a peak in the molecular ring of about 4 times its local value. This suggests that the cool phase is more abundant there, and colder, than it is locally. The distribution of cool-phase temperatures is derived in three different ways. The naive, " spin temperature" technique overestimates the cloud temperatures, as expected. Using two alternative approaches, we get good agreement on a histogram of the cloud temperatures, Tcool, corrected for blending with warm-phase gas. The median temperature is about 65 K, but there is a long tail reaching down to temperatures below 20 K. Clouds with temperatures below 40 K are common though not as common as warmer clouds (40-100 K). Using these results, we discuss two related quantities, the peak brightness temperature seen in emission surveys and the incidence of clouds seen in H I self-absorption. Both phenomena match what would be expected based on our measurements of 〈k〉 and T cool.